One of the most interesting conclusions from special relativity can be best explained by an Einstein quote “the distinction between past, present, and future is a stubbornly persistent illusion”. That seems to drip with the possibility of time travel, but while fascinating, we are prevented from exploiting it by...
Best answer: One of the most interesting conclusions from special relativity can be best explained by an Einstein quote “the distinction between past, present, and future is a stubbornly persistent illusion”. That seems to drip with the possibility of time travel, but while fascinating, we are prevented from exploiting it by perhaps the most sacred law in physics - causality cannot be violated.
Einstein’s quote comes from the relativity of simultaneity between different inertial frames in special relativity. Two causally connected events in spacetime happen at different times in different reference frames. If the events are simultaneous in one reference frame, they are not simultaneous in another reference frame. If the events are simultaneous in the ‘present’ of one reference frame, they are not simultaneous in the present of other reference frames. One of the events will occur in the past or future of another reference frame depending on whether it is moving toward or away from the other frame. This negates the idea that there is any notion of absolute past or absolute future, e.g. things are done for everybody and haven’t happened for everybody. We cannot exploit this in relativity because we cannot MOVE FAST enough to violate the causal sequence of the two spacetime events. Special relativity prevents that.
General relativity is a different story. There can be extreme changes in the the geometry of spacetime due to extreme densities and distributions of matter and energy. There are solutions of general relativity, closed time curves and wormholes, that need to be discussed in the context of time travel.
Closed time curves are a similar idea to a closed loop in space, but the closed loop is time not space. These pop up in some solutions of General Relativity such as the Godel metric, which is a contrived metric for a rotating universe, with a contrived distribution of matter and energy. One of the issues with general relativity is that it is ignorant about certain other laws of physics. For instance, general relativity is time symmetric, it does not know about entropy, quantum mechanics, and it has issues with energy conservation. So, while closed time curves are allowed, they are likely non-physical solutions prevented by other laws of physics such as entropy and energy conservation.
Wormholes are always a fun, controversial subject. They are also allowed solutions to general relativity and I’m not going to go into details since I haven’t seen anybody on YA that understands wormhole metrics so it would be a waste of time to go into any detail. Suffice to say that solutions of General Relativity can be found that include wormholes, but the wormholes connect two separate ‘flat’ spacetime regions. General Relativity cannot tell us whether those flat regions are two separate regions of our universe or regions of two separate universes. So, in order to preserve causality, IF somehow a traversable wormhole could be constructed, the requirement for preservation of causality would mean that the two regions are in separate universes and no time travel could actually occur, and hence no causality violations.
1 week ago